Coated metal wires are used for numerous purposes, such as general purpose electrical wire, magnetic wire, building wire, cables, lead or hook-up wire, and the like. One very important application for coated metal wires is as a winding for electrical equipment such as motors, transformers, magnetic coils, relays and the like.
Insulated electric wire for use in windings for electrical equipment (magnet wire) has been conventionally manufactured by passing a filament or strand of the bare metal wire through a bath of liquid enamel (a solution of resin material in a solvent therefor) and through an oven for driving off the solvent from the enamel and/or curing the resin, leaving a resin coat on the metal wire. This technique necessitates several successive passages of the wire through the liquid enamel bath of insulating material in order to obtain the required thickness of the coating (insulating layer). The application of several coats of the liquid enamel to the metal wire from solution accounts for almost all of the magnetic wire being manufactured today.
The equipment used in applying multiple layers of liquid enamel to wire is highly complex and expensive. In addition to this, the cost of the solvent expended in applying multiple layers of the resinous material from solution is usually very significant. The use of such solvents can also bring about certain environmental pollution and safety problems. In some cases the use of solvent based liquid enamels has necessitated the installation and maintenance of various types of pollution control equipment.
The use of extrusion coating in manufacturing coated wire is desirable because it eliminates the need for using a solvent. Applying coatings of resinous material by extrusion is substantially less common than applying coatings from solution and conventional extrusion processes are extremely limited. Coatings of four mils and less are either extremely difficult to apply or impossible to apply by conventional extrusion techniques. Also, the number of materials which are normally applied by conventional extrusion processes are extremely limited. Polyvinyl chloride, polyethylene, polypropylene and various elastomeric rubbers comprise almost all of the materials conventionally applied by extrusion techniques. These materials are not used in a true magnet wire application because they do not possess the requisite properties. For example, in an electrical winding the magnet wire is insulated to provide voltage, mechanical, and thermal protection between the turns on the coils. In contrast, these materials are conventionally used in lead wire or hook-up wire applications which must protect against the full input line voltage of an electrical device. Consequently, extrusion is used only in the conventional production of cables, building wire, and lead or hook-up wire.
Conventional extruding equipment for coating wires is not without problems. Conventional extruders include a centering die, a material reservoir and a sizing die. The centering die mechanically centers the wire in the sizing die, the sizing die determines the exterior dimensions of the coated wire filament. The primary problem associated with extrusion apparatus is the wear on the centering die. Since the centering die is used to center the wire filament within the sizing die, the centering die must be finely adjusted to achieve a concentric coating and must be replaced periodically due to the wear resulting from the contact between the wire filament and the die. Centering dies are generally made of hardened steel; but because of the high amount of wear that occurs, diamond centering dies have been considered, but are not widely accepted.
The development of an improved method for manufacturing magnet wire having all of the benefits of an extrusion process and none of its disadvantages would be highly desirable. Such a method would lower the cost of the machinery required to manufacture such wire and would eliminate the need for solvent, lower manufacturing costs, conserve raw materials and energy, eliminate the need for any pollution control apparatus, require less expensive and simpler machinery than is now conventional, and allow for the continuous processing of the wire from the stage at which it is drawn to final shipment at relatively high speeds.
The process of coating metal wires with thermoplastic resins by extrusion is not new. For example, U.S. Pat. No. 4,165,957 discloses an apparatus for coating wire with very high melting-point thermoplastics in an extrusion process. The apparatus disclosed therein includes what is essentially a "pressure" cross-head die. U.S. Pat. No. 4,145,474 describes a general process wherein thermoplastic polycondensates are extruded onto and around metal wires. This patent indicates that the thermoplastic polycondensates which can be utilized include (1) linear polyesters of high molecular weight, such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthoate; (2) polyarylesters; (3) linear aliphatic polyamides having high molecular weight; (4) aramides; and (5) sulfur polymers, such as polyphenylene sulfide. U.S. Pat. No. 4,391,848 also describes a process for coating magnet wire with "flowable materials" such as nylon, polyethylene terephthalate, polyethylene, polycarbonates, polysulfones, epoxys, and polyetheramides, which utilizes a specific type of coating die. This coating die has an entrance opening larger than its throat portion wherein its entrance opening and throat portion are interconnected by a converging interior wall to define a die cavity. This die cavity is at least partially filled with the flowable material when this process is used for coating magnet wire.